I suppose we'll find out how as they develop epic more but its always fun to speculate. The easiest way I can think of is to have people set some, or all if they wish I guess, of their family npcs as guards that can take and store weapons. You can make it so players can too if they want but I don't foresee many people lining up for that. We can set npcs to farm and craft afaik so I don't see this being an issue.
My impression so far is that you will be able to set some sort of rules applying to the land you "own" and build your city on (this area should expand with every building you construct in close proximity to another building already existing, imo).
With "rules" I mean things like...
... you can only "enter" the city unarmed, leading to your weapons being automatically taken from you by the server when you walk through the gates.
... the "tax" rate they mentioned concerning the amount of labor force every crafting citizen has to give to the local ruler (which is also automatically subtracted from the amount you produce or the trades you do).
... who can construct buildings in separate sections of your city.
... how many of your family members (assuming your whole family lives in this place) can be used as soldiers during times of war.
... which kind of goods you are allowed to produce within the city borders.
... and so on
No one is needed to enforce these rules within a city because they are set by the ruler and restrict or allow certain gameplay mechanics while you are in the town or city. On top of that, if your local ruler is only a small lord belonging to a bigger kingdom, the king or marshal might also set some rules applying to his whole kingdom / area of influence (at least to all cities or villages belonging to his dominion).
This is the kind of "metagame" associated with being an empowered person in OKAM... That's how I understood it so far.
This is necessary due to the fact that Taser is right, there will definitely not be enough people willing to do the boring RP tasks needed to carry all this out with players only.